1. Field of the Invention
The present invention relates to a display apparatus and, in particular, to a display apparatus having enhanced light extraction efficiency and good photopic contrast.
2. Description of the Related Art
Organic EL devices (light emitting devices) are characterized by the fact that they utilize spontaneous light emission from thin films and are used in a new type of flat-panel displays. Such an organic EL device includes a pair of electrodes constituted by a cathode and an anode and an organic compound layer formed between the pair of electrodes. Light emission from such an organic EL device employs a principle in which a molecule emits light upon transition from an excited state to the ground state. Specifically, electrons are injected from the cathode and holes are injected from the anode into the organic compound layer to thereby generate excitons in the light emitting layer in the organic compound layer. The excitons bring molecules in the light emitting layer into the excited state. The molecules emit light upon transition from the excited state to the ground state. Such a light emitting layer is composed of a light emitting material such as a fluorescent organic compound, a phosphorescent organic compound, or a quantum dot.
To enhance the visibility of organic EL devices outdoors, it is important to improve the photopic contrast of organic EL devices. A means for improving the photopic contrast is to suppress reflection of ambient light and it has been proposed to place a circular polarizer on the light extraction side of such an organic EL device. Another means for improving the photopic contrast is to enhance the light emission efficiency to thereby increase the maximum luminance of light emission. Japanese Patent Laid-Open No. 11-283751 discloses an organic EL device including a periodic structure (diffraction grating) disposed in an upper portion or a lower portion of an organic compound layer (that is, the light extraction side of the device or the side opposite to the light extraction side) for the purpose of reducing confinement of light generated in the device and enhancing the light emission efficiency.
However, when an organic EL device has a configuration in which these two features are combined, that is, when a periodic structure is provided in an organic EL device equipped with a circular polarizer, the following problem is caused. The periodic structure disturbs the circular polarization state of ambient light having entered the device through the circular polarizer and the function of suppressing reflection of ambient light is degraded, the function being provided by the circular polarizer.
This problem is not restricted to organic EL devices and light emitting devices including circular polarizers and periodic structures such as diffraction gratings also suffer from such a problem.